What We Believe by Cannot Proveedited by John BrockmanThe Free Press £9.99, pp266

John Brockman is the PT Barnum of popular science. He has always been a great huckster of ideas. In the Sixties he got an MBA and then made his first fortune selling psychedelia to corporations, turning on marketing execs with 'multi-kinetic happenings' and showing them how their profits could levitate. These days he acts as literary agent for many of the world's greatest minds, including Richard Dawkins and Daniel Dennett, and achieves for some of them the kind of publishing advances that it takes great mathematicians to compute. The universe may be infinite, but Brockman still insists on his 15 per cent of it.

In order to help these floggable ideas to keep germinating, Brockman has set up a kind of global online Royal Society, called the Edge. The Edge promotes what he calls the Third Culture, a marriage of science and philosophy and even poetry, an alternative to CP Snow. This cross-fertilising club, the most rarefied of chat-rooms, has its premises on his website www.edge.org, which presents monthly interviews and debates with many of the world's foremost thinkers, who also happen to be, more often than not, his clients.

As a favour, perhaps, or because they are genuinely intrigued or flattered to be in the company of their peers, every year a good number of these thinkers respond to a question posed by Brockman. In 2004 the question was: 'What do you believe is true, even though you cannot prove it?'

The 100-odd responses to this question make up this book, which is compelling and repetitive by turns. Mostly, the correspondents are concerned with the two great time-honoured questions: is there life anywhere else in the universe? And, are we anything more than flesh and blood? Almost everyone, it appears, believes there is life elsewhere, but hardly anyone has truck with any notion of anything like an individual soul, or even a tangible 'I'. 'Mind', as John Dewey suggests, has become a verb, not a noun.

Despite this denial of a single self, there are plenty of formidable egos on display. Craig Venter, the man who tried to patent our DNA, the Book of Life, and make his fortune from it, believes that all life is a 'panspermic event'; that primordial microbes were sent in a directed rocket ship from an alien galaxy to colonise the earth, a big bang if ever there was one. We, in turn, are constantly pansperming, launching a zillion microbes back into space.

Others are slightly more circumspect. Kenneth W Ford is the retired director of the American Institute of Physics. He believes in the existence of life elsewhere because 'chemistry seems to be so life-striving'.

He also offers a couple of sobering thoughts. Microbes have occupied Earth for at least 75 per cent of its history; intelligent life for about 0.02 per cent of it.

He believes: 'Mars will be found to have harboured life and harbours life no more.' The effect of this on mankind, perhaps only a couple of years away, might be psychologically dramatic; more dramatic than 'Copernicus, Galileo, and Newton; perhaps even more so than the discovery of life elsewhere in the galaxy.'

Brockman characterises our times as the Age of Certainty, in that we are comfortable with materialism and Darwinism, and believe that science could unlock all of the world's mysteries, given enough time; our own version of monkeys and typewriters. Ian McEwan, the novelist and Edge thinker, in his introduction, suggests that we are not so far from the 'old Enlightenment dream of a unified body of knowledge, when biologists and economists draw on each other's concepts and molecular biologists stray into the poorly defended territory of chemists and physicists'.

This faith is striking, and it fervently ignores the contrary tide of superstition and fundamentalism across the world. Even as they are thinking the unthinkable, however, many of Brockman's correspondents, largely Americans, seem also to be arming themselves against a return to an age of uncertainty in which creationism and intelligent design hold sway in the public mind. Enlightenment and infinity are hedged around, these days, by the Holy Neocon Empire.